Problem(s) Encountered:

TCS failed.  Telescope refused to slew, throwing an error and turning off the tracking.  On-site observers power-cycle the main TCS box in the computer room at Eric''s suggestion, but then the dome and the dec axis both refused to home (HA and focus both homed normally).  The problem was intractable, so the night was lost.

Solutions:

Came in this morning and found the system to still be failing as it was last night.  Long and short of it is that the dec motor electronics (driver?) had failed.  The display was flashing ‘err’, which I took as a good sign that it was experiencing an error.  I flipped the knife switch, killing power, waited, then brought it back online.  The display went back to a somewhat more familiar ‘r  0’.  I then tried homing the dec axis, and the telescope moved in dec.  But then I got an error that it had timed out.  Display started indicating roughly HA of 0 and Dec of +43ish.  I could see however that the telescope was pretty close to zenith, so… I tried homing dec again.  Same thing.  Figuring something in the TCS might need to initialize correctly now that the dec driver was back online, I once more power cycled the TCS.  When that was done, I tried homing everything again.  This time, the dec appeared to home properly, but I never saw the message that it was done homing.  I know I didn’t miss it because I saw the done homing message for HA still being displayed.  Instead, I got some message about track rates for HA and dec being set to 0.00000.  I did however see that the display indicated HA of 0 and dec of +32.  After a few minutes of waiting, I finally assumed it had in fact completed the home for dec, and so continued on, homing the dome, which worked fine.  I then drove the telescope and dome around and made sure it was functioning as expected.  It was.  The last test I performed was to turn on tracking, let it track for a short time, then slew to another position, then let it track some more.  I was ensuring that tracking remained on (last night, the problems all started when John noticed that track would shut off when a slew was issued, even though the slew likely never happened).  Once I verified that the tracking was stable, I stowed the telescope and for one last time, attempted to home the dec.  This time it went through the procedure and ended by informing me that it had in fact homed the dec.  So now, all appears to be working again.

This does not explain to me why the electronics interface failed in the first place.  This hopefully was just a random failure.  The other oddity was that the HOME button on the TCS GUI would almost immediately go red when trying to home the dome last night.  Indeed, it did this to me as well this morning.  The way I got around this was to instead click on the DOME button, and demand that it drive to the home position.  Ever since doing that, it has homed correctly.